doc: Fix iterating over the Mock object in Python 3
doc: Fix iterating over the Mock object in Python 3

Building the documentation for salt.modules.snapper fails with Python 3:

```
$ make -C doc html
[...]
WARNING: [autosummary] failed to import 'salt.modules.snapper': no module named salt.modules.snapper
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 109 source files that are out of date
updating environment: [config changed] 1608 added, 17 changed, 0 removed
reading sources... [100%] topics/yaml/index
doc/ref/modules/all/index.rst:20: WARNING: failed to import snapper
doc/ref/modules/all/index.rst:20: WARNING: toctree references unknown document 'ref/modules/all/snapper'
WARNING: autodoc: failed to import module 'salt.modules.snapper'; the following exception was raised:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/sphinx/ext/autodoc/importer.py", line 152, in import_module
    __import__(modname)
  File "salt/modules/snapper.py", line 72, in <module>
    if SNAPPER_DBUS_OBJECT in bus.list_activatable_names():
TypeError: argument of type 'Mock' is not iterable
```

Fix the Mock object to be iterable (as it was intended to).

Signed-off-by: Benjamin Drung <86a6db991ec5c1a96f98c92b98df9d547d7c8c84@cloud.ionos.com>

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt